Optical communication system transmits information from a transmitter having a light source to a receiver having a photodetector via an optical fiber. A semiconductor light source comprises a semiconductor laser diode chip (1), a photodiode chip (2), a header (3) on which the laser diode chip (1) and the photodiode chip (2) are mounted, leads (4) fixed to the header (3), a cap (5) with a window fitted on the header (3), and a spherical lens (6) fixed to the window of the cap (5). The laser diode chip (1) emits modulated light beams bearing signals. The photodiode chip (2) monitors the output power of the laser diode by detecting backward light emitted from the laser diode chip (1). The lens (6) converges the light beams from the laser diode chip (1) on a core of an optical fiber (9) with high efficiency. The leads (4) are connected to both electrodes of the laser diode chip (1) and the monitoring photodiode chip (2) for introducing a driving current onto the laser diode chip (1) and for extracting a photocurrent from the monitoring photodiode chip (2). The cap (5) has another object of sealing an inner space airtightly besides of supporting the lens (6).
The output power of a semiconductor laser diode is varied owing to functional degeneration and change of the atmospheric temperature, even if the driving current is kept to be constant.
Therefore, the monitoring photodiode chip (2) is installed behind the laser diode chip (1). The laser diode chip (1) emits light beams both to a front side (7) and a rear side (8). The front beams are converged on the optical fiber (9) to transmit information signals. The rear beams enter the photodiode chip (2) which monitors the output power of the laser diode chip (1). Since the ratio of the front beams to the rear beams is constant, irrespective of the fluctuation of the output power of the laser diode, the photocurrent of the photodiode is proportional to the front light beams bearing information signals. Therefore, if the driving current of the laser diode is controlled so as to keep the photocurrent of the monitoring photodiode to be constant, the front beams of the laser diode can be maintained to be constant.
However, current digital optoelectronic communication transmits digital signals by representing "0" and "1" by changing the output of the laser diode in square waves. An increase of sending information per second raises the frequency of the change of the light intensity emitted from the laser diode (1). For example, when one billion bit / sec (10.sup.9 bit/sec) of information signals are transmitted, the length of a bit signal is shorter than 1 us (nanoseconds). Such high speed transmission will incur the difficulty of deformation of square pulses. In the case, the rising time or falling time of a pulse becomes the focus of attention.
The case where the response time is as short as the order of 1 ns will be considered now. The light intensity emitted from the laser diode changes in one ns. The photocurrent of the monitoring photodiode must change in accordance with the rapid change of the light intensity. If the delay of the function of the monitoring photodiode were left untouched, the following malfunctions would occur.
Owing to the delay of response of the photodiode, the rising of a square pulse would be sometimes delayed and the falling of a square pulse would also be delayed to the change of the light intensity. At the earliest stage of a light pulse, no photocurrent would flow in the photodiode, although the light enters the photodiode. At the moment, the feedback circuit would make an erroneous judgement that no light would now be emitted from the laser diode, and would increase the driving current for the laser diode, because the photodiode did not seem to feel light beams entering therein. The erroneous increase of the driving current of the laser diode would lead to surplus of the light intensity beyond the allowed scope. On the contrary, at the final stage of a pulse, some photocurrent would still flow in the photodiode owing to the delay of the photocurrent generation, although light emission of the laser diode has already been ended. The feedback circuit would again misunderstand the state of the light emission and would reduce the driving current below the pertinent level, since it still receives a photocurrent from the monitoring photodiode. In such a case, a reverse current may flow in the laser diode, depending on the structure of the electronic circuits. The excess current at the early stage and the reverse current at the final stage of a pulse may injure or destruct the laser diode. Such malfunctions result from the delay of the monitoring photodiode.
Suppression of the delay of the monitoring photodiode will solve the difficulty of malfunctions of the driving circuit. The reason why the delay of the photodiode chip is induced is now briefly explained. A conventional photodiode chip (2) has a semiconductor substrate (11), epitaxial layers (12) deposited on the substrate (11), a diffusion region (13) produced at the center of the epitaxial layers (12), an annular electrode (14) formed on the diffusion region, and a planar electrode (15) formed on the bottom side of the substrate (11). If the substrate is n-type, the diffusion region is p-type. If the substrate is p-type, the diffusion region is n-type. The substrate (11) and the diffusion region (13) have reciprocal conduction properties. A dish-like boundary between the diffusion region (13) and the epitaxial layers (12) is a pn-junction (18). The central part enclosed by the annular electrode (14) is now called a receiving region for light. Light beams (16) which enter the receiving region will generate rapid photocurrents owing to the strong electric fields acting on the region. The strong electric fields carry electrons and holes excited by the light beams with high velocities to the electrodes.
The peripheral part outside the pn-junction (18) is now called a non-receiving region. If the light beams (20) entered the non-receiving region, slow photocurrents would be induced, because weak electric fields would drag the electrons and the holes slowly into the electrodes. These slow flows of carriers in the non-receiving region cause the delay of the photodiode. Thus, elimination of the light entering the non-receiving region will prevent the photodiode function from delaying.
In order to avoid the delay of the monitoring photodiode, a prior light source has strictly determined the position of the monitoring photodiode (2) so as to forbid light beams entering the non-receiving region. The position of the laser diode (1) must also be strictly determined in order to prevent the rear light beams from entering the non-receiving region of the photodiode (2).
Another prior light source employs the feedback circuit which controls the average of the light intensity of the laser to suppress the delay of the photodiode monitoring signals. In this case, the photocurrent of the photodiode is averaged by using, e.g. a capacitor. Since the driving currents are determined on the basis of the averaged photocurrents, the delay of the photodiode response becomes insignificant. The delayed parts are cancelled by taking an average.
However, such an average photocurrent method would suffer from another difficulty. Transmitting signals have various kinds of sequences of levels "0" and "1" in digital communication. Sometimes only level "0" continues, other times only level "1" continues. Otherwise, levels "1" and "0" repeat in turn many times. Therefore, the averages of light intensity would vary to a great extent according to the behavior of the sequences of levels.
For example, a simplified case wherein the level "0" is designated by 0 mW of light power and the level "1" is designated by 1 mW of light power is now assumed to clarify the difficulty. When only the low level "0" continues in the signal, the average of light power becomes 0 mW. When the high level "1" continues alone, the average of light power becomes 1 mW. When both levels "0" and "1" repeat in turn with the same probability, the average of light power is 0.5 mW. The driving circuit has adjusted to supply currents to the laser diode for ensuring average light power of 0.5 mW. If only the low level "0" continues, the diving circuit will supply surplus current for keeping the average light power to be 0.5 mW. But in this case, the light power must maintain 0 mW, because the signals are a sequence of "0"s. On the contrary, if the high level "1" continues alone, the driving circuit will send insufficient currents for keeping the average light power to be 0.5 mW. In this case, the light power must hold 1 mW, because the signals are a sequence of "1"s. The method of the average power control is suitable for the reciprocating sequence of "0" and "1" of signals ( mark rate is 0.5 ) but is erroneous for long sequences of only "0"s or only "1"s.
A prior method of the average power control has made efforts to maintain the mark ratio to be about 0.5 at all times by giving digital signals a scramble or eliminating the time of no signal transmission to avoid malfunctions.
However, a recent tendency of optoelectronic communication requires such a flexible system that can deal with the time change of information quantity. The average power control cannot satisfy the requirement. The main stream of the communication system is turning from the average power control to a peak-holding type which can control individual pulse power. The tendency also demands a monitoring photodiode which is suitable for the peak-holding type control.
Semiconductor light sources are indispensable for optoelectronic communication. But the difficulty and high price of current light sources impede wide prevalence of the optoelectronic communication. One purpose of this invention is to provide a semiconductor light source which can be assembled without difficult adjustment of positions of laser diode and the photodiode. Another purpose of this invention is to provide a semiconductor light source which is suitable for the peak-holding control. The other purpose of this invention is to provide a semiconductor light source which will contribute rapid pervasion of optoelectronic communication systems by supplying inexpensive light sources.